The present invention relates to a semiconductor device in which semiconductor chips are mounted at a high density, a method for manufacturing an electronic equipment such as a portable information terminal in which the semiconductor device is mounted, an electronic equipment, and a portable information terminal.
Electronic equipments including a portable information terminal such as a mobile phone must be reduced in size, and in particular in thickness. Various methods for mounting a semiconductor chip have been proposed for such reduction in size. A particularly useful method among the proposed methods is a method in which two semiconductor chips are mounted on the top surface and the back surface of a wiring board so as to face each other. There are two methods as such a method for mounting the semiconductor chips on the top surface and the back surface of a single wiring plate.
The first method is a method in which a mounting/connecting substrate is provided between a wiring board and a mounting board, and the wiring board is connected to the mounting board through the connecting substrate, enabling a semiconductor chip on the back surface to be mounted without contacting the mounting board.
FIG. 11 is a cross-sectional view of a conventional electronic equipment mounted by such a method, for example, a cross-sectional view of a conventional electronic equipment disclosed in Japanese Laid-Open Publication No. 7-240496. As shown in FIG. 11, an electronic equipment is formed with a semiconductor device mounted on a mounting board 108. The semiconductor device includes semiconductor chips 101a, 101b respectively mounted on a top surface and a back surface of a single wiring board 103. The wiring substrate 103 is bonded to, and electrically connected to, the mounting board 108 through a connecting substrate 106. More specifically, electric connection between electrodes 104 of the wiring board 103 and electrodes 116 of the connecting substrate 106 as well as electric connection between the electrodes 116 of the connecting substrate 106 and electrodes 109 of the mounting board 108 are both implemented with solder bumps 107. The thickness of the semiconductor chip 101b is herein set to a value smaller than the total thickness of the connecting substrate 106 and the solder bump 107 so that connection to the mounting board is not hindered. Note that the wiring substrate 103 is electrically connected to the first and second semiconductor chips 101a, 101b through electrodes 102. Moreover, the respective connections between the wiring board 103 and the first and second semiconductor chips 101a, 101b and the connection between the wiring board 103 and the connecting substrate 106 are sealed with a sealing resin 105.
The second method is a method in which a recess is formed at the back surface of a wiring board, and a semiconductor chip is fittingly mounted in the recess. This method also enables a semiconductor chip on the back surface to be mounted without contacting a mounting board.
FIG. 12 is a cross-sectional view of a conventional electronic equipment mounted by such a method, for example, a cross-sectional view of a conventional electronic equipment disclosed in Japanese Laid-Open Publication No. 10-79405. As shown in FIG. 12, a recess is formed at the back surface of the wiring board, and the semiconductor chip 101b is fittingly mounted in the recess. Mounting the semiconductor chip 101b in the recess enables connection between solder bumps 107 and electrodes of a mounting board (not shown) to be achieved without being disturbed by the semiconductor chip 101b. Note that the wiring substrate 103 is electrically connected to the first and second semiconductor chips 101a, 101b through electrodes 102. Moreover, the respective connections between the wiring board 103 and the first and second semiconductor chips 101a, 101b are sealed with a sealing resin 105.
The first and second methods thus enable the wiring substrate to be mounted on the mounting board so that the semiconductor chip on the back surface does not contact the mounting board.
However, the first method necessitates the use of an expensive connecting substrate, increasing the manufacturing costs of the electronic equipment. Moreover, since the wiring substrate is bonded to the mounting board through the connecting substrate, the total thickness is increased due to insertion of the connecting substrate, hindering reduction in thickness.
The second method necessitates formation of a recess in the wiring board, which requires special man-hour. Accordingly, this method also increases the manufacturing costs of the electronic equipment.
One possible way to mount on the mounting board the wiring substrate having a semiconductor chip mounted on both surfaces thereof without using any connecting substrate and without forming any recess in the wiring substrate is to reduce the thickness of the semiconductor chip 101b as much as possible. In this case, however, reduction in thickness of the semiconductor chip 101b reduces the rigidity, resulting in degraded reliability and the like.
Moreover, it is now assumed that the semiconductor chip 101b and the mounting board 108 have a small gap therebetween. In this case, even if the semiconductor chip 101b does not contact the mounting board 108 when mounted, the mounting board 108 may be subjected to bending or torsional stresses by external pressure and the like generated when the product is in use. This would cause the surface of the semiconductor chip 101b to contact the mounting board 108, damaging the semiconductor chip 101b. 
Moreover, it is now assumed that the semiconductor chip 101b and the mounting board 103 have a small gap therebetween. In this case, even if the semiconductor chip 101b does not contact the mounting board 108 when mounted, the mounting board 108 may be subjected to bending or torsional stresses by external pressure and the like generated when the product is in use. This would cause the surface of the semiconductor chip 101b to contact the mounting board 108, damaging the semiconductor chip 101b. 
The present invention is made to solve such conventional problems as described above, and it is an object of the present invention to provide a low-cost semiconductor device including a semiconductor chip mounted on both surfaces of a wiring board without degrading electric characteristics, a method for manufacturing an electronic equipment, an electronic equipment, and a portable information terminal.
A semiconductor device according to the present invention includes: a wiring substrate including electrodes on a top surface and a back surface thereof; projecting electrodes formed on one surface of the wiring substrate so as to have a prescribed height; a semiconductor chip having a thickness smaller than the height of the projecting electrodes and mounted on the one surface of the wiring substrate so as to be electrically connected to the electrodes of the wiring substrate; and an electronic component having a thickness larger than that of the semiconductor chip and mounted on the other surface of the wiring substrate as to be electrically connected to the electrodes of the wiring substrate so that the wiring substrate is warped to be recessed at the one surface.
In such a semiconductor device, the wiring substrate is warped toward the side having the projecting electrodes (bumps) (i.e., warped so as to be recessed on the side having the projecting electrodes). Therefore, the semiconductor chip will not contact the mounting board when the wiring substrate having the semiconductor chip mounted thereon is mounted on the mounting board, enabling the semiconductor chip to be mounted without being damaged. Moreover, neither the use of a connecting substrate nor formation of a recess in the wiring substrate is required, enabling the semiconductor device to be manufactured inexpensively.
In order to warp the wiring substrate toward the side having the projecting electrodes, a linear expansion coefficient of the electronic component may be made smaller than that of the wiring substrate. This is because the degree and direction of the warping are almost determined by the electronic component and the wiring substrate. Preferably, a value of the linear expansion coefficient of the electronic component is equal to or less than that of the semiconductor chip.
In the case where the longitudinal and lateral dimensions of the semiconductor chip are the same, the wiring substrate is warped to the same degree in the longitudinal and lateral directions into a bowl shape. In the case where the longitudinal and lateral dimensions of the semiconductor chip are different, the wiring substrate is warped into a bowl shape, but the warping degree is different between the longitudinal and lateral directions. Too large warping would no longer be able to be absorbed by the projecting electrodes provided on the mounting board, and also would degrade electric characteristics at the electronic component or the semiconductor chip. Therefore, the wiring substrate is preferably warped so that a difference in level between a central portion and a peripheral portion of the wiring substrate is equal to or less than 100 xcexcm. Moreover, since the wiring substrate must be warped, the difference in level between the central portion and the peripheral portion of the wiring substrate is more preferably in the range of 5 xcexcm to 100 xcexcm, and is preferably in the range of 10 xcexcm to 40 xcexcm in view of actual manufacturing.
The electronic component may either be a single semiconductor chip or a lamination of a plurality of semiconductor chips. In order to reliably improve the rigidity and suppress degradation in electric characteristics, the electronic component desirably has a thickness of 0.3 mm or more. On the other hand, excessively increasing the thickness of the electronic portion would result in increased thickness of the semiconductor device, which goes against the trend toward reduction in thickness. Therefore, the thickness of the electronic component is desirably about 0.65 mm or less. In order to improve the rigidity more reliably and prevent degradation in electric characteristics, the electronic component may be larger than the semiconductor chip in two-dimensional size.
For example, a printed board, a printed board having a fine wiring layer formed at its surface by means of an epoxy resin and plating, or a resin substrate formed from a polyimide resin and a conductor is preferably used as the wiring substrate.
Another semiconductor device according to the present invention includes: a wiring substrate including electrodes on a top surface and a back surface thereof; projecting electrodes formed on one surface of the wiring substrate so as to have a prescribed height; a semiconductor chip having a thickness smaller than the height of the projecting electrodes and mounted on the one surface of the wiring substrate so as to be electrically connected to the electrodes of the wiring substrate; and an electronic component mounted on the other surface of the wiring substrate so as to be electrically connected to the electrodes of the wiring substrate, the electronic component having a thickness larger than that of the semiconductor chip and having a linear expansion coefficient smaller than that of the wiring substrate.
In such a semiconductor device, the linear expansion coefficient of the electronic component is smaller than that of the wiring substrate, whereby the wiring substrate is warped toward the side having the projecting electrodes. Therefore, the semiconductor chip will not contact the mounting board when the wiring substrate having the semiconductor chip mounted thereon is mounted on the mounting board, enabling the semiconductor chip to be mounted without being damaged. Moreover, neither the use of a connecting substrate nor formation of a recess in the wiring substrate is required, enabling the semiconductor device to be manufactured inexpensively.
As described above, a resin substrate is preferably used as the wiring substrate, and the electronic component preferably has a thickness of 0.3 mm or more.
A method for manufacturing an electronic equipment according to the present invention is a method for manufacturing an electronic equipment by mounting a semiconductor device on a mounting board with projecting electrodes interposed therebetween, the semiconductor device having the projecting electrodes and a semiconductor chip both provided on a mounting side thereof, and having an electronic component provided on a side opposite to the mounting side, wherein the semiconductor chip has a thickness smaller than a height of the projecting electrodes, the electronic component has a thickness larger than that of the semiconductor chip, and the semiconductor device is mounted on the mounting board with the projecting electrodes interposed therebetween by aligning with the mounting board the semiconductor device warped so as to be recessed on the mounting side, and pressing the semiconductor device against the mounting board with the semiconductor device being in the warped state.
In such a manufacturing method of an electronic equipment, the wiring substrate is mounted on the mounting board while being warped so as to be recessed on the side facing the mounting board. Therefore, even if there is only a small difference between the height of the projecting electrodes provided on the wiring substrate and the thickness (height) of the semiconductor chip, the semiconductor chip is mounted without contacting the mounting board. Thus, the semiconductor chip can be prevented from being damaged due to contact with the mounting board.
In order to warp the wiring substrate toward the side having the projecting electrodes, a linear expansion coefficient of the electronic component may be made smaller than that of the wiring substrate. In this case, since the wiring substrate has a larger linear expansion coefficient than the electronic component, the wiring substrate is subjected to larger shrinkage than the electronic component when cooled. Accordingly, the wiring substrate is shrunk more than the electronic component, whereby the wiring substrate is warped into a bowl shape so as to project toward the electronic component. Preferably, a value of the linear expansion coefficient of the electronic component is equal to or less than that of the semiconductor chip.
This semiconductor device may be manufactured by a manufacturing process including the steps of bonding the electronic component to the wiring substrate with heating and then cooling the resultant wiring substrate, thereby mounting the electronic component on the wiring substrate such that the wiring substrate is warped so as to be recessed on the mounting side, mounting the semiconductor chip on the mounting side of the wiring substrate, and forming the projecting electrodes on the mounting side of the wiring substrate.
An electronic equipment according to the present invention includes: a mounting board; a wiring substrate including electrodes on a top surface and a back surface thereof and mounted on the mounting board with projecting electrodes interposed therebetween, the projecting electrodes having a prescribed height; a semiconductor chip having a thickness smaller than the height of the projecting electrodes and mounted on a surface of the wiring substrate facing the mounting board such that the semiconductor chip is disposed in a space between the mounting board and the wiring substrate and electrically connected to the electrodes of the wiring substrate; and an electronic component mounted on a surface of the wiring substrate opposite to that facing the mounting board such that the electronic component is electrically connected to the electrodes of the wiring substrate, the electronic component having a thickness larger than that of the semiconductor chip and having a linear expansion coefficient smaller than that of the wiring substrate.
A portable information terminal according to the present invention includes: a housing; a mounting board mounted in the housing; a logic LSI chip mounted on the mounting board; a wiring substrate including electrodes on a top surface and a back surface thereof and mounted on the mounting board with projecting electrodes interposed therebetween, the projecting electrodes having a prescribed height; a memory or logic LSI chip having a thickness smaller than the height of the projecting electrodes and mounted on a surface of the wiring substrate facing the mounting board such that the memory or logic LSI chip is disposed in a space between the mounting board and the wiring substrate and electrically connected to the electrodes of the wiring substrate; and an electronic component mounted on a surface of the wiring substrate opposite to that facing the mounting board such that the electronic component is electrically connected to the electrodes of the wiring substrate, the electronic component having a thickness larger than that of the semiconductor chip and having a linear expansion coefficient smaller than that of the wiring substrate.
In such an electronic equipment and a portable information terminal, the semiconductor chip such as memory or logic LSI chip can be mounted on the mounting board at a prescribed distance without contacting the mounting board, enabling the semiconductor chip to be mounted without being damaged. Accordingly, an electronic equipment and a portable information terminal can be provided that are advantageous in terms of reduction in size and weight.
As described above, a resin substrate is preferably used as the wiring substrate, and the electronic component preferably has a thickness of 0.3 mm or more.